Constant Bhp Kill Methods

There are three 'constant bottom-hole pressure' kill methods in common use today which are:

  • Driller's Method
  • Wait & Weight Method (also known as the 'Engineer's Method')
  • Concurrent Method.

These three techniques are very similar in principle, and differ only in respect of when kill mud is pumped down.

In the Driller's method, the kill is split into two circulations. During the first, the kick fluid is circulated without changing the mud weight; once the kick is out, the mud is weighted up and pumped around the well on the second circulation.

The Wait and Weight method achieves both of these operations simultaneously. Kill mud is prepared before starting the kill, and the kick fluid is circulated out while this mud is circulated into the well.

In the Concurrent method, a compromise is adopted between these two methods. The kick fluid is circulated out while the mud being circulated in, is weighted up in stages, towards the kill weight.

Of the three methods, the Wait & Weight method is the most popular, and the one recommended for use in most instances. It produces lower well pressures than either of the other methods, with an evident benefit to safety. Each of the three methods is now described in detail.

1.3.1 The Driller's Method

In the Driller's method, the kick is circulated out of the hole using the existing mud weight. The mud weight is then raised to the required level and circulated around the well.

Two complete circulation's are thus required, as a minimum, for this method. Since it deals separately with the removal of the kick and the addition of kill weight mud, it is generally considered to be the simplest of well control methods, and it requires least arithmetic. However, this results, in the well being circulated under pressure for a relatively long time, possibly the longest of the three methods, with an increased possibility of choke problems. Also, the annular pressures produced during the first circulation are higher than produced with any other method.

CAUTION: BECAUSE VERY HIGH ANNULAR PRESSURE MAY ARISE WHEN KILLING A GAS KICK WITH THIS METHOD, CARE SHOULD BE TAKEN. ANNULAR PRESSURE WILL BE AT A MAXIMUM IMMEDIATELY BEFORE GAS ARRIVES AT SURFACE, AND CASING BURST PRESSURE LIMITATIONS MAY BE CRITICAL. FOR THIS REASON, THE DRILLER'S METHOD IS NOT GENERALLY RECOMMENDED.

In general, this method is most used on small land rigs where the Driller may have little help and limited equipment. A variant of this method is also used in some workover operations.

In addition the simplicity of the Driller's method makes it useful when only limited information is available about the well conditions.

To summarise:

  • FIRST CIRCULATION: Pump the kick out of the well, using existing mud weight.
  • SECOND CIRCULATION: Pump kill weight mud around the well.

a) Advantages of driller's method

  • MINIMUM ARITHMETIC
  • MINIMUM WAITING AROUND TIME - CAN START KILL AT ONCE
  • MINIMUM INFORMATION REQUIRED.

b) Disadvantages of driller's method

  • HIGHEST ANNULAR PRESSURE PRODUCED
  • MAXIMUM WELL UNDER PRESSURE TIME
  • LONGEST 'ON-CHOKE' TIME.
  1. Procedure for driller's method; See Figure 2 (a) - (f)
  2. The well is closed in and the information recorded.

FIRST CIRCULATION

2) If a slow circulating rate pressure, P SCR, has been taken, then calculate the pressure required on the drillpipe for the first circulation of the well.

This is:

Initial Circulation = Slow Circulation Rate + Shut In Drillpipe Pressure Pressure Pressure or:

ICP = PSCR + SIDPP

  1. Open the choke about one quarter, start the pump and break circulation; then bring the pump up to the Kill Rate.
  2. While the Driller is bringing the pump up to the Kill Rate, the choke operator should operate the choke so as to keep the casing pressure at or near the closed in casing pressure reading.
  3. Once the pump is up to the Kill Rate, the choke operator should transfer his attention to the drillpipe pressure gauge and adjust the choke to maintain the INITIAL CIRCULATING PRESSURE on the drillpipe pressure gauge.
  4. The INITIAL CIRCULATING PRESSURE is held constant on the drillpipe pressure gauge by adjusting the choke throughout the whole of the first circulation, until all of the kick fluid has been circulated out of the well. The pump rate must also be held constant at the KILL RATE throughout this period.
  5. Once the kick is out of the hole, shut the well in and mix up the kill mud weight required.

KlllW(pigg)tMud = Old Mud Weight + [SIDPP (psi)* TVD(ft) * 0.052]

NOTE 1: This is a kill weight mud to balance formation pressure. It is the lowest possible mud weight which will 'kill' the well. Once the well is dead, it will be necessary to increase the mud weight further to provide a trip margin.

NOTE2: Some operators prefer to continue circulating the well while kill weight mud is being mixed. There is no theoretical reason why this should not be done, though it does result in further wear and tear on equipment under pressure - in particular the choke.

Methods Well Kill
Figure 2

SECOND CIRCULATION

  1. Once the kill mud is ready, open the choke about one quarter, start the pump and break circulation. Then bring the pump up to the kill rate.
  2. While the Driller is bringing the pump up to the kill rate, the choke operator should operate the choke so as to keep the casing pressure steady at the same pressure as when closed in.
  3. While the drillpipe is being filled with heavy mud, hold the casing pressure constant. The drillpipe pressure will go down as the drillpipe is being slugged with the heavier mud. In practice, if all the kick was properly removed in the first circulation, the choke should not need to be touched once the pumps are steady at the Kill Rate, until kill mud reaches the bit.

Once the kill mud reaches the bit, the pressure held on the drillpipe is just that required to circulate the mud around the well. This is the slow circulating rate pressure, increased slightly for the extra mud weight.

Final Circulating _ SlowCirculatirg Kill Mud Weight

Pressure RatePressure OriginalMud Weight

The drillpipe pressure starts dropping below the initial circulating pressure, as the kill mud starts down the drillpipe, reaching the final circulating pressure when the kill mud reaches the bit. Thereafter the drillpipe pressure is held at the final circulating pressure by controlled opening of the choke, as the kill mud moves up the annulus.

A graph showing how drillpipe pressure drop from the initial to the final circulating pressure is shown in Figure 3 and this can be used as a guide to the drillpipe pressure required. The drillpipe pressure should drop according to the graph, as kill mud goes to the bit, without the choke being moved.

Drillpipe Pressure

Figure 3 - Graph of Drillpipe Pressure as Kill Mud is Pumped

Pump Stroke

Figure 3 - Graph of Drillpipe Pressure as Kill Mud is Pumped

Because of the possibility that the annulus may not be circulated to completely clean during the first circulation, it may be preferable to work out how the drillpipe pressure should vary, as kill mud is pumped in and around the well. This will allow the drillpipe pressure to be used throughout, so eliminating the possibility of small gas bubbles in the annulus producing misleading information.

The following graphs depict the variations in pressure during the well circulation.

Driller Method Well Control
Graph 1 - First Circulation

SECOND CIRCULATION

Start Finish

Start Finish

Methods Well Kill

CIRCULATING PRESSURE

Methods Well Kill
Graph 2 - Second Circulation

d) Determination of initial circulating pressure

If no slow circulating rate pressure has been taken, then the initial circulating pressure can be determined using the start-up procedures described in the circulations of the Driller's method.

Where the casing pressure has been held constant while the pumps are brought up to a kill rate, the drillpipe pressure reading will be the initial circulating pressure.

The procedure consists of:

  1. Noting casing pressure reading.
  2. Adjusting pumps to new kill rate. Adjusting choke to hold casing pressure constant at the value noted.
  3. As soon as the driller has the pumps settled on the new rate, return to the drillpipe pressure gauge. Note this new reading as the circulating pressure for the new pump rate and maintain this.

NOTE: This procedure is satisfactory at any time during a kill providing the mud weight in the drill string is not changing during the process. It is however preferable to maintain pump rate constant as much as possible. Any decision to change pump rate should be taken early.

1.3.2 Wait and Weight Method

The 'Wait and Weight' is sometimes referred to as the 'Engineers Method' or the 'One Circulation Method'. It does, at least in theory, kill the well in one circulation.

This is the preferred method used by most operators and recommended by many well killing experts. Its principal advantage is that it provides the lowest annular pressures during the circulation of the kill, making it the safest of the commonly used kill methods.

Once the well is shut in and pressures stabilised, the shut in drillpipe pressure is used to calculate the kill mud weight. Mud of the required weight is made up in the mud pits. When ready, kill mud is pumped down the drillpipe. At commencement enough drillpipe pressure must be held to circulate the mud, plus a reserve equivalent to the original shut in drillpipe pressure. This total steadily decreases as the mud goes down to the bit, until with kill mud at the bit, the required pressure is simply that needed to pump kill mud around the well.

The choke is adjusted to reduce drillpipe pressure while kill mud is pumped down the string.

With kill mud at the bit, the static head of mud in the drill pipe balances formation pressure. For the remainder of the circulation, as the influx is pumped to the surface, followed by drill pipe contents and the kill mud, the drillpipe pressure is held at the final circulating value by choke adjustment.

a) Advantages of the wait and weight method

  • LOWEST WELL BORE PRESSURES, AND LOWEST SURFACE PRESSURES -this means less EQUIPMENT STRESS.
  • MINIMUM 'ON-CHOKE' CIRCULATING TIME.

b) Disadvantages of the wait and weight method

  • CONSIDERABLE WAITING TIME (while weighting up) - GAS MIGRATION?
  • IF LARGE INCREASES IN MUD WEIGHT REQUIRED, THIS IS DIFFICULT TO DO UNIFORMLY IN ONE STAGE.

c) Procedure for the wait and weight method

The Wait and Weight method uses the same calculations already described for a drillpipe pressure schedule. The calculations are:

Kill Mud Weight = OngmalMud Wdght + [ ^ ^ ^ ^ (PPg) (PPg)

At the start of the circulation, with kill mud:

Initial Circulating SlowCirculatirg Rate Shut In Drillpipe Pressure _ Pressure + Pressure

Once the capacity of the drill string is calculated, it is possible to draw a graph showing how drillpipe pressure varies as kill mud is pumped down to the bit.

Once kill mud is ready, the start-up procedure is as previously described.

The choke is cracked open, the pump started to break circulation, and then brought up slowly to the Kill Rate.

While the Driller brings the pump up to the Kill Rate, the choke operator works the choke so as to keep the casing pressure at or as near as possible to the closed in casing pressure reading.

When the pump is up to the Kill Rate, the choke operator transfers to the drillpipe pressure gauge, adjusting the choke if necessary to achieve the INITIAL CIRCULATING PRESSURE on the drillpipe pressure gauge.

As the kill mud proceeds down the drillpipe, the drillpipe pressure is allowed to drop steadily from the Initial Circulating Pressure to the Final Circulating Pressure, by choke adjustment.

Where the kick is a small one, at or near the bottom of the hole, the drillpipe pressure tends to drop of its own accord as the kill mud moves down. Little or no choke adjustment is required.

Only in cases of diffused gas kicks with gas far up the annulus will significant choke adjustments be needed during this period.

After kill mud has reached the bit, the drillpipe pressures is maintained at the Final Circulating Pressure, until the kill mud returns to surface.

As with the Driller's method, this Final Circulating pressure is held constant as long as pump rate is held constant at the selected value. If, for any reason, the pump rate is felt to be wrong, it can be changed using the same procedure described previously. However, pump rate changes should be avoided, where possible.

While the pump rate is adjusted, the casing pressure is held steady by adjusting the choke. Once the pump is stabilised at its new speed, the revised circulating pressure is read from the drillpipe gauge. If a gas influx is very near to the surface, adjusting pump rate by holding a steady casing pressure may significantly increase the bottom hole pressure. This is due to the rapid expansion of gas near the surface. Alterations in pump rate are to be made early on!

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Responses

  • LAURA
    How to establish ICP and scr using the drillers method?
    8 years ago
  • swen trommler
    What are the advantages of the concurrent method in well control?
    8 years ago
  • BERHANE
    Which circulating method results in highest pressure on annulus?
    8 years ago
  • aamu
    How choke maintain drillpipe pressure?
    7 years ago
  • Torin
    How to calculate scr pressure in well control?
    7 years ago
  • nathanial
    How to work out kill rate circulation pressure formula?
    7 years ago
  • Elanor
    How to keep bottom hole pressure constant while killibg well?
    2 years ago
  • Kerry
    What pressure will kept constant while killing a well?
    2 years ago
  • TOM
    What happen to bottomhole pressure during kill operation if the sidpp decreases?
    1 year ago
  • Daniel
    What happens to bottom hole pressure if you hold casing constant?
    1 year ago
  • Wolfgang
    Why SCIP is higher than SIDPP in drilling?
    1 year ago
  • essi
    Why do we use the drillpipe gauge for pressure readings during well control?
    11 months ago
  • rosamunda
    What happens to btm hole pressure when you open the chok to much?
    9 months ago
  • crystal
    How to maintain constant bottom hole pressure?
    8 months ago
  • Semrawit
    How much back choke pressure to hold while killing well?
    7 months ago
  • travis
    When adjusting choke will I see casing pressure move first?
    7 months ago
  • GIUSEPPA
    When kill mud is pumping what happened?
    5 months ago

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